This invention relates generally to motors have heaters which are used to prevent condensation on the motor windings when the motor is not running and, in particular, to a motor having a thermostatically controlled space heater.
It has been recognized that condensation tends to develop on a motor winding after the motor has increased in temperature during an operating cycle and the motor cools after the operating cycle. In the prior art, this problem has been partially addressed by space heaters used to maintain the temperature of the motor winding above ambient temperature when the motor is not running. In particular, the prior art has mounted space heaters adjacent to the motor windings to generate heat which is transferred to the motor windings. Such space heaters are operated when the motor is not running. Due to the high surface temperatures developed by such space heaters, a thermally-insulated layer is positioned between the space heater and the motor winding. Although this tends to inhibit heat transfer to some extent, it prevents direct contact of the space heater with the motor winding. Because of the high surface temperature of the space heater, direct contact between the space heater and the motor winding could damage the motor winding by melting winding insulation or causing other thermal deterioration of the winding.
For some motor applications in hazardous environments, maximum surface temperatures must be limited in order to prevent or minimize the possibility of auto-ignition of gases, vapors or dust that may be present. In such environments, prior art motors with space heaters may not be used or useful because of the high surface temperatures developed by the space heaters.
Therefore, there is a need for a motor having a space heater with low maximum surface temperatures which would prevent thermal damage to the motor windings and which could be used in hazardous locations. There is also need for such motors which are efficient and reduce the power requirements to power the space heater when the motor is not running.